BackChapter 1: Biology – The Study of Life
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Biology: The Study of Life
Defining Life: The Five Fundamental Characteristics
Biology is the scientific study of life, and all living organisms share five fundamental characteristics that distinguish them from non-living matter.
Cells: All organisms are composed of membrane-bound cells, which are the basic units of life.
Replication: All organisms are capable of reproduction, ensuring the continuation of their species.
Information: Organisms process hereditary information encoded in genes and respond to environmental information.
Energy: All organisms acquire and use energy to maintain their internal order and support life processes.
Evolution: Populations of organisms are continually evolving, leading to the diversity of life observed today.
Theories in Biology
Scientific Theories and Their Role
A theory in science is an explanation for a broad class of phenomena or observations that is supported by a wide body of evidence. This differs from the everyday use of the word, which often means speculation or guesswork.
Cell Theory: Addresses what organisms are made of and their origins.
Theory of Evolution by Natural Selection: Explains how organisms are related and how they change over time.
Chromosome Theory of Inheritance: Describes how hereditary information is transmitted from one generation to the next.
Life is Cellular and Replicates through Cell Division
Discovery of Cells
The discovery of cells was a pivotal moment in biology. In the 1660s, Robert Hooke and Anton van Leeuwenhoek used microscopes to observe cells for the first time, leading to the formulation of the cell theory.
Robert Hooke: Observed cork tissue and coined the term "cells."
Anton van Leeuwenhoek: Observed single-celled organisms, which he called "animalcules."
Cell Theory and the Rejection of Spontaneous Generation
The cell theory states that all organisms are made up of cells and that all cells come from preexisting cells. This challenged the idea of spontaneous generation, which posited that organisms could arise from non-living matter.
Louis Pasteur's Experiment: Demonstrated that cells arise only from preexisting cells, not by spontaneous generation.
Cell Division and Common Lineage
Cells replicate through division, and all cells in a multicellular organism are descended from a common ancestor. This supports the idea of a shared lineage among all living things.
Additional info: Modern evidence suggests that life originated from non-life through chemical evolution early in Earth's history.
Life Processes Information and Requires Energy
Chromosome Theory of Inheritance
The chromosome theory of inheritance, proposed by Sutton and Boveri, states that genetic information is encoded in genes located on chromosomes. In the 1950s, it was established that chromosomes are composed of DNA, which is the hereditary material.
Genes: Segments of DNA that code for cell products.
The Central Dogma of Molecular Biology
The central dogma describes the flow of genetic information within a cell: DNA is transcribed into RNA, which is then translated into protein. Proteins determine the physical traits of an organism.
DNA → RNA → Protein
Genetic Variation and Heredity
DNA is copied with high accuracy during cell division, but occasional mutations (changes in DNA sequence) can lead to changes in proteins and, consequently, in an organism's traits. These heritable variations are the basis for the diversity of life.
Energy and Nutritional Needs
All living organisms require energy to drive the chemical reactions necessary for life. They have two fundamental nutritional needs:
Acquiring chemical energy, often in the form of adenosine triphosphate (ATP).
Obtaining molecules that serve as building blocks for DNA, RNA, proteins, and other cellular components.
Additional info: The way organisms acquire energy is a key factor in the diversification of life.
Life Evolves
Evolution and Natural Selection
Evolution is the change in the characteristics of a population over time. Darwin and Wallace proposed that species are related by common ancestry and that characteristics can be modified from generation to generation (descent with modification).
Mechanisms of Evolution: Natural Selection
Natural selection is the process by which certain heritable traits increase an individual's fitness (ability to produce offspring) in a particular environment. Over time, these traits become more common in the population, leading to evolutionary change.
Conditions for Natural Selection:
Individuals vary in heritable characteristics.
Certain traits increase reproductive success in specific environments.
Speciation: Occurs when populations diverge to form new species.
Example: Finches on the Galápagos Islands developed small, pointed beaks as an adaptation to eat small, soft seeds, increasing their fitness and prevalence in the population.
The Tree of Life and Evolutionary History
Phylogeny and the Tree of Life
The tree of life is a family tree that depicts the evolutionary relationships among species, with a single ancestral species at its base. Phylogeny refers to the actual genealogical relationships among all organisms.
Analyzing Genetic Variation
Biologists compare DNA and RNA sequences among organisms to determine evolutionary relationships. Fewer sequence differences indicate a closer relationship.
Organism | DNA Sequence |
|---|---|
Land plant | A – T – A – T – C – G – A – G |
Green algae | A – T – A – T – G – G – A – G |
Brown algae | A – A – A – T – G – G – A – C |
Conclusion: Green algae are more closely related to land plants than to brown algae.
The Phylogenetic Tree of Life
The phylogenetic tree is used to show relationships between species. Branches that share a recent common ancestor represent closely related species, while those that do not are more distantly related. The tree of life is estimated from genetic data.
Major Groups of Life: Eukaryotes and Prokaryotes
The tree of life indicates three major groups of organisms:
Eukaryotes: Organisms with a nucleus (Domain Eukarya).
Prokaryotes: Organisms without a nucleus, divided into Bacteria and Archaea.
